Introduction

In the study of metallic materials it is often to analyze the phases exist and grain size in the structure. Details of the structure of metals are not readily visible through naked eyes, but grain structures in metals may be seen with the aid of microscope. Metal characteristics such as grain sizes, effect of heat treatment, and carbon content of steels may be determined by studying the micrograph. For this purpose, the metal used in the metallurgical examination must be prepared and polished carefully before a good microscopic image can be seen. It is important to ensure that the surface is totally flat and smooth before the microstructure observation. Any irregularity will appear as a dark image and may create confusion to the observer who attempts to analyze a structure. In order to obtain this smooth flat surface, several preparatory steps are required: Cutting, Molding, Grinding, Polishing and Etching.

Cutting is a process done to cut the sample metal to appropriate size to have an easy holding for grinding and polishing processes. The cutting tool used is abrasive cut off machine which used proper cutting blade for the material of sample. Molding process is a process done when the sample is too small and hard to handle by hand for grinding and polishing processes. Molding consist of 2 types which is Hot Mounting and Cold Mounting which differs by the sensitivity of heat and pressure of the sample used. For Hot Mounting the substance used to form the mold is Phenolic Powder and took a short time to prepare rather than the Cold Mounting process. The Cold Mounting process used a mixture of resin and took about 24 hour to form the mold.

Grinding process is main process to get the microstructure of the sample. This process is done to remove the rough surface of the sample. The sample need to undergo a long grinding process using different grades of sand paper. The process must start from lower grade of sand paper until the highest grade. Polishing is a process carried out to get the best surface of sample where the sample shining and turn to be a mirror-like surface. This process is carried out on a cloth covered rotating wheels and involve with a different grade of abrasive powders, suspended in a water solution. The sample need to be hold firmly on the polishing wheel with a specific period of time and to get better result, use the higher grade of abrasive powder. Etching process is a crucial process for this lab experiment. This process is done to bring out the microstructure of the polished sample. The polished surface of the sample is subjected to the chemical action of an appropriate reagent depends on the material of sample. This process need to be done in specific period of time correctly depend on the material. If the etching time is too long, it may burnt the surface of the sample and can not be used to see the microstructure under the microscope. The sample need to start over from the grinding process to get the new surface.

A metallurgical microscope has a system of lenses (objectives and eyepiece), because of its ability to study objects with highly polished like metals, a metallurgical microscope is different from other microscopes. When a mirror-like metal surface has been obtained, the structure can be observed with the aid of optical microscope. Schematic diagram of an optical microscope is shown in the right figure. The optical microscope magnifies an image by sending a beam of light through the object as seen in the schematic diagram. The condenser lens focuses the light on the sample and the objective lenses (10X, 40X…, 2000X) magnifies the beam, which contains the image, to the projector lens so the image can be viewed by the observer.